Ultra high frequency impedance measuring apparatus



- 1943- M. -w. SCHELDORF ,337,934

ULTRA HIGH FREQUENCY IMPEDANCE MEASURING APPARATUS! Filed Sept; 15, 1942 lj' u I l 4,

Z: u? f Inventor- Marvel ScheIdor-fi His tater-neg.

REAC TAIVL'E IN (II-1H5 Patented Dec. 28, 1943 ULTRA HIGH FREQUENCY IMPEDANCE MEASURIN G APPARATUS Marvel W. Scheldorl, Schenectady, N. Y., assignor to General Electric Company,

New York a corporation of Application September 15, 1942, Serial No. 458,377

'5 Claims.

My invention relates to a method and apparatus by means of which measurements can be made for the accurate determination of the values of unknown impedances in ultra high frequency circuits, both as regards the resistance and plus or minus reactance component values thereof.

Considering the difficulties generally involved in making measurements and calculations pertaining to high frequency circuits, the apparatus used and the calculations necessary for the determination of the unknown circuit constants according to my invention are relatively simple.

According to my invention I make use of transmission lines, one being a supply line and the other of which is adjustable and includes the unknown impedance in series. The measurements used are the length of the variable line and the relation between two voltages, one being the voltage across the adjustable transmission line and unknown impedance connected in series and the other voltage being that across the supply line taken one-quarter wave length from the first voltage measurement point.

The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of my invention reference is made in the following description to the accompanying drawing in which Fig. 1 represents an explanatory diagram of the transmission line apparatus and connections employed in my invention. Fig. 2 shows a sectional view of a concentric ca-ble transmission line arrangement of my invention which is the more practicable and preferred form. Fig. 3 represents a cross-section of the concentric cable arrangement of Fig. 2 taken on lin 33. Fig. 4 represents an explanatory vector diagram for a resonant condition of adjustment of the variable impedance transmission line section. Fig. 5 is the corresponding diagram for a non-resonant adjustment and Fig. 6 is a resonance curve which is useful in obtaining accurate measurement values.

Referring now to Fig. 1, I0 represents an ultra high frequency oscillator for generating the ultra high frequency source of supply employed for the measurement circuit. For example, if it is desirable to measure the impedance of an unknown impedance device at 300 megacycles, then a 300 megacycle source of supply is used. The oscillator is coupled to the local end of and feeds a transmission line section II which extends to a voltmeter V2 at the remote end. Between voltmeter V2 and the source I0 a second voltmeter VI is connected across line section II. V2 and VI will also be used hereinafter to designate the voltages measured by the corresponding voltmeters. Voltmeter VI is placed one-quarter wave length away from voltmeter V2 towards the source I0. This can be done by adjusting position of VI until W is a maximum. This creates the very desirable situation that for a given constant value of VI the voltage V2 is directly proportional to any impedance connected in parallel with V2. This follows from a familiar transmission line relationship, namely that for two points one-quarter wave length apart, the current at one point is directly proportional to the voltage at the other point.

Extending beyond voltmeter V2 transmission line section I2 which has an adjustable length 1. That is, a short circuiting connector I3 is provided which is adjustable along line I2 so that the effective length I from voltmeter V2 to the terminal short circuit I3 may be varied. The unknown impedance Zr which is to be measured is connected in series with line I2 between voltmeter V2 and the beginning of line section I2, as indicated. Because of ultra high frequency transmission line properties the position of short circuit I3 may be adjusted so that the line can be made to have a terminal impedance which has, for practical purposes, no resistance and either inductive or capacitance reactance of any value from zero to infinity. Hence with the series circuit combination of Zr and I2 in parallel with V2, it is possible by adjusting I3 along line I2 to balance out the reactance component in the unknown impedance is a second Zr. This condition will be indicated when the voltag relation is a minimum which corresponds to the lowest impedance across V2. When such adjustment is reached, the reactance of the line section I2 will be equal in magnitude and opposite in sign to the reactance component Xx of Zrc. The reactance of line I2 corresponding to a given value of 1, can be readily determined mathematically from its geometry. It is equal to Z0 tan Bl, where Z0 is the unit impedance determined from the effective spacing of the conductors of line I2, 1 its length which is measured and B the electrical angle of the line determined by the frequency used or is the length of line I2 in electrical degrees per unit of length Z. 20 tan Bl is thus known or readily determinable. Having found the reactance component of 21:, the resistance component can also be determined by another adjustment of line I2. The relations may be pictured vectorially in Figs. 4 and 5.

In Fig. 4 the vector Za: representing the unknown impedance, is the hypotenuse of a right angle triangle, the other two sides of which comprise the resistance component Ra: of the unknown and the reactance component Xx of the unknown (shown ositive for example, to represent an inductance). Thus, Z$=R$+7'X:l:, where 7' indicates the right angle vector relation of R1: and X35.

From the principle that the ratio of is proportional to the impedance connected in parallel with V2, it follows that the voltage relation V1 is proportional to the magnitude of Rm-l-J(X:c+Zo tan Bl) When we adjusted l to make a minimum we adjusted line II to a condition of resonance where Z tan Blr=-Xa:. In this equation Zr has been used .to indicate the length of line I2 for the resonant condition. The value Z0 tan Blr is pictured in Fig. 4 by the length of the vertical line below vector Rm. That is. it is equal to X11: but is negative and thus indicates a capactive reactance.

To find Ra: we now proceed as follows: I is changed to produce a non-resonant condition which, of course, increases the voltage relation The minimum value at resonance will be designated and the increased ratio In the condition represented in Fig. 4 the apparent impedance connected in parallel with voltmeter V2 is Ra: because the reactance components Xx and Z0 tan Blr have been balanced out. Hence the minimum voltage ratio V 2 V]- f is roportional to Rm. When I is changed to produce a non-resonant condition,

Kg V1 is increased and its relation to V1 may be represented in Fig. 5 by the dotted line.

Xx is no longer balanced because the reactive impedance of line I2 has been increased to a nonresonant condition value Z0 tan 31:20 tan BZr+AX=-X:r+AX Xa: was determined and hence is known. Zo tan Bl can be determined as before, taking the new measurement Z. Hence AX can be determined.

In Fig.5

v1 lazwnx (1L2 Rx V1 The ratio V2) (a V 2 (v1 which I will designate E can be determined from the voltmeter readings In practice the lines II and I2 are made concentric in accordance with good high frequency practice and are largely placed one inside of the other to reduce the overall dimensions to a minimum as represented in Figs. 2 and 3. Also, the intermediate concentric conductor I5 serves as one conductor of both lines II and I2 of Fig. 1.

In Figs. 2 and 3 I have three concentric conductors, an inner conductor I4, an intermediate tubular conductor I5 and an outer tubular conductor I6. Conductors I5 and I6 serve as the line I I of Fig. 1 with the oscillator source of supply IIl adjustably inductively coupled thereto through the loop conductor parts I1 and I8.

' connections for voltmeter VI.

The connections I! and I8 form the source end of line II. Conductors I4 and I5 form the line I2 of Fig. 2 with the short circuiting device I3 adjustable by an adjusting device I9. A scale 20 is provided, calibrated in millimeters to read off the length Z of line I2. The unknown impedance Z0: is connected between conductors I6 and M at the lower open end of the concentric cable assembly. This point forms the remote end of one line and the beginning of the adjustable line. Voltmeter V2 is connected across conductors I5 and I6 also at the lower open end of the concentric line assembly. Between conductors I5 and I6 an adjustable concentric brush assembly comprising a brush 2| bearing on the inner surface of cable I6 and a brush 22 bearing on the outer surface of cable I5 comprise the A suitable adjusting device 23 provides means'for adjusting this brush assembly along the cables to make the distance thereof from voltmeter connection V2 one-quarter wave length. It is desirable to provide a scale 25 to indicate the position of the VI connection.

The precautions usual with high frequency measuring circuits are necessary. The high frequency circuit connections are made as short as possible and are shielded. A resistance 28 is shunted across voltmeter connection VI to terminate or match the line at this point. Vacuum tube rectifier voltmeters are used, small shielded tubes being used with short high frequency connections to the line conductors. At VI in Fig. 2 such a shielded tube is indicated while at V2 the main tube connections are indicated. Direct current proportional to the voltages measured elongated slot 3| therein to bring out the D. C.

connections from vacuum tube voltmeter Vl.

By the arrangement described it is possible to confine the dimensions of the line apparatus to practicable limits. For example, using 300 megacycle measurement frequency correspond-' ing to a wave length of one meter, the length of conductor section l6 may be 32 inches and the overall length of conductors II and I5 may be 65 inches. For these dimensions the inner conductor ll may be inch in diameter; conductor l5 may have an inner diameter of inch and an outer diameter of one inch. Conductor I 6 may have an inner diameter of 1% inches and an outer diameter of 2 inches. For such dimensions the surge impedance of line I I is 24.3 ohms and that of line I2 is 41.6 ohms at 300 megacycles.

The use of the apparatus will now be reviewed,

using actual test data. First, the 300 megacycle energy constant voltage is applied. The VI brushes are then adjusted until the voltmeter reading ratio V1 is a maximum, which occurs when VI .is onequarter wave length from V2. The unknown impedance Za: is now connected in line l2. Slider I3 is now adjusted until the voltage ratio 71 is a minimum. This is or should be a condition of resonance. However, where, as for the Zn: here assumed, the resonance condition is not sharp but is rather broad it will be difficult to adjust this setting with high accuracy unless a resonance curve of line I2 is plotted. Hence, a series of additional voltmeter ratio readings are taken with larger and smaller values of Z than the one which appears to give the minimum ratio These additional readings are well oif the resonance condition and a resonance curve is plotted as shown in Fig. 6.

Actual readings and calculations made were as follows:

appears to be a minimum. At position I scale 20 is read and I found to be 738.5 mm. Xrc is now calculated from the formula Xa:=Zo tan Blr. Actually X.7: is taken from a table which is calculated on the basis of the physical dimensions of line 12 and for the length of line 738.5 is found to be -20 ohms; If the determination of this value is correct AX=0. The value 20 ohms is plotted on a chart the abscissa of which is :Xa: ohms and the abscissa is the ratio of see Fig. 6.

Next we adjust slider I3 until the scale 20 is 624.7.

V1 is found to be 6.18 and X:r is calculated to be --92. This is plotted on the chart, point 2. Similarly we determine -X.'z: for the other values of 1 given and plot the results at points 3, 4 and 5. When a sufiicient number of such points are plotted we can draw in the resonance curve with a high degree of accuracy. We find from the correct curve that Xa: is 1'l.5 ohms and that X7: is therefore 17.5 ohms.

Corrected ratio of from the curve is 5.25. AX is obtainable directly from the curve, Fig. 6, for example AX for point 2=92-17.5='74.5; for point 4=80+17.5=97.5.

AX R2; 4

is now obtainable and is found from an average of the four readings to be 122 ohms. Hence Z.7:=(122+J 17.5) ohms.

What I claim as new and desire to secure by Letters Patent of the United States is:

-1. In a method of determining the reactance component of an unknown impedance at an ultra high frequency, the steps which consist in impressing a constant high frequency voltage on a transmission line at one end. measuring the reading on voltage along such line at points designated V2 and VI, A; wave length apart, one of which points V2 is at the remote and of such line, thereby obtaining a maximum voltage ratio, impressing the voltage at the remote end of such transmission line across the unknown impedance and a second transmission line in series, the second transmission line having known electrical characteristics and an adjustable length determined by a short circuit, adjusting the length of the second transmission line until the reactance component of the unknown impedance is balanced out, such adjustment being obtained when the voltage ratio becomes a minimum, and measuring the length of the second transmission line as thus adjusted for the purpose of determining such balanced out reactance from known char acteristics of such second transmission line.

2. In a method for determining the reactance component and the resistance component of an unknown impedance at anultra high frequency,

the steps which consist in impressing a constant ultra high frequency voltage on a transmission line at one end, measuring the voltage along such transmission line at two points. designated V2 and VI, wave length apart, one point V2 being at the remote end of such transmission line, thereby obtaining a maximum l VI voltag ratio, impressing the voltage at the remote end of such transmission line across the unknown impedance and a second transmission line in series, the second transmission line having known electrical characteristics and an adjustable length determined by a short circuit, adjusting the length of the second transmission line until the reactance component of the unknown impedance is balanced out, such adjustment being obtained when the voltage ratio becomes a minimum designated then measuring such length for the purpose of determining the reactance component of the second transmission line from its known characteristics and its length as thus adjusted, then changing the length of the secondtransmission line to obtain a higher voltage ratio designated 22 V1 measuring the new length for the purpose of determining the reactance component ofthe second transmission line for this new adjustment in order to solve for the resistance component of the unknown impedance according to the formula /E 1 where AX represents the difference between the reactance components of the second transmission line for its two mentioned adjustments and 3. Apparatus for use in determining the reactance and resistance components of unknown impedanqes at ultra high frequencies comprising a transmission line, a constant voltageultra high frequency sourc for supplying such transmission line at one end, means for simultaneously measuring the voltages at the remote end and wave length distance from the remote end of said transmission line, a second transmission line having known electrical characteristics, means for connecting the unknown impedance and the second transmission line in series across the remote end of the first mentioned transmission line, a short circuit determining the length of said second transmission line the position of which is adjustable to vary the length of the second transmission line, and means for measuring the length of the second transmission line.

4. In an ultra high frequency impedance measuring system, a rod and two tubes concentrically arranged,'the tubes comprising a first transmission line and the inner tube and rod comprising a second transmission line, an ultra high frequency source of voltage arranged to supply the first transmission line at one end, means for measuring the voltages at the remote end of the first transmission line and at a point A wave length from the remote end, means for connecting the second transmission line in series with an unknown impedance across the remote end of the first transmission line, and means for varying the length of the second transmission line comprising a conductor connected between said rod and inner tube and adjustable along the same.

5. Apparatus for determining impedance values at ultra high frequencies including a pair of transmission lines comprising a concentric cable structure having an inner rod, an intermediate tube and an outer tube, the outer tube being slotted to facilitate voltage measurements between it and the intermediate tube, the tubes comprising a first transmission line and the rod and intermediate tube comprising a econd transmission line, means for impressing an ultra high frequency voltage on one end of the first transmission line, rectifier type vacuum tube voltmeters for simultaneously measuring the voltage of the first transmission line at the remote end thereof and at a point wave length from the remote end, provisions for connecting an impedance to be measured and the second transmission line in series across the remote end of the first transmission line, and means comprising a conductor slider bridged between the rod and intermediate tube for adjusting the length of the second transmission line.

MARVEL W. SCHELDORF. 

